Which disease has the higher morbidity and mortality rate than any other source of gi bleeding?

The right answer is: Bleeding oesophageal varices

Bleeding oesophageal varices occur when swollen veins (varicose veins) of the lower esophagus rupture and bleed.
Oesophageal varices may leak blood and eventually rupture. This can lead to serious bleeding and life-threatening complications, including death. When that happens, it's a medical emergency.

Mitochondria produce atp, a high energy compound. atp is used for all sorts of enzyme-mediated reactions, such as making proteins/nucleotides/etc, certain compounds cross the cell membrane, muscles move, etc.  without this source of energy, a cell will die. it will not be able to repair itself, defend against pathogens, etc.